Purpose Medication non-adherence to treatment regimens can severely impact the mortality of patients afflicted with breast cancer.The purpose of this study was to identify factors that contribute to non-adherence to endocrine therapy in breast cancer treatment plans. Methods Thirty-two women with a breast cancer diagnosis were surveyed by pharmacists and pharmacy students to identify the patient- related factors (e.g. patient personal beliefs, education level), drug-related factors (e.g. patient drug allergies), socio-economic factors (e.g. patient ability to pay for the medication) and healthcare system factors (e.g. poor patient-healthcare provider relationship) that may impact non-adherence to endocrine therapy in breast cancer treatment plans. Medication adherence rates were measured using the Medication Adherence Rating Scale (MARS-8) system. Associations between adherence rate scores and clinical variables (e.g. age, tobacco use, alcohol consumption, cost of treatment, education level, personal beliefs, drug allergies, patient/provider relationship, adverse events) were carried out using Spearman Correlation, T-Test, Mann-Whitney U Test, and X2 tests. A p value of ≤ 0.05 was considered statistically significant. Results Our study found that 59% of survey respondents were non-adherent to their endocrine therapy in breast cancer treatment plans. Drug allergies ( p = 0.000069), patient ability to pay ( p = 0.005), poor personal beliefs about the prescribed therapy (p = 0.009), low education level ( p = 0.025), adverse drug events ( p = 0.026), and poor patient-provider relationship ( p = 0.05) were found to play a role in patient non-adherence to treatment. Conclusions Our study found that drug- (e.g. allergies), socio-economic (e.g. patient ability to pay), and patient-related factors (e.g. personal beliefs) are the strongest predictors of adherence among breast cancer patients undergoing endocrine therapy. These findings support the need for a better relationship between breast cancer patients and their healthcare providers, including drug experts such as pharmacists.